Juha Kangasluoma

Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation. In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.

Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation. In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.

Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation. In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.

Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation. In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.

Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation. In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.

Isoprene-derived oxygenated organic molecules (IP-OOM) drive new particle formation in the upper troposphere (Shen et al., 2024; Curtius et al., 2024), but the mechanisms that produce them, and the role of photochemistry in shaping their distribution, remain poorly constrained. These products are both hard to quantify, owing to isobaric and isotopic interferences that conventional time-of-flight mass spectrometers cannot resolve, and hard to interpret, as their formation pathways cannot be established from the measured product distribution alone. We address both with a combined measurement and modelling framework, which we demonstrate through chamber and flow reactor experiments. On the measurement side, we optimise a high-resolution Q Exactive Plus Orbitrap mass spectrometer, coupled to a bromide multi-scheme chemical ionisation inlet (the Br^–-MION2-Orbitrap), by tuning three operating parameters that change individual-species signals by 10–90 %. Benchmarked side by side against a Br^–-MION2-APi-TOF in chamber experiments, it agrees to within a factor of two for the oxygenated oxidation products while resolving isobaric interferences the time-of-flight instrument cannot. This comparison also shows that the Orbitrap’s apparent non-linearity at low concentrations, sometimes regarded as an intrinsic limitation, is rather an artefact of post-acquisition data processing: averaging that assigns zero intensity to non-detections biases the signal low near the detection limit. Disregarding the non-detections gives a conservative upper-bound detection limit of ∼1.3×10⁵ cm⁻³, comparable to that of the APi-TOF. On the modelling side, we extend MARFORCE from simple inorganic chemistry to comprehensive organic mechanisms of hundreds of species coupled to transport, and add a continuous, spatially distributed OH source; the same model can also run as a well-mixed box model for chamber experiments. Applying the framework to the OH oxidation of 1,2–ISOPOOH in the flow reactor, we detect several abundant products absent from the current isoprene mechanism, and find that photolysis of the hydroperoxide-bearing products, a previously underappreciated process, shapes the product distribution. These experiments show how pairing molecular-level measurement with a model that resolves chemistry and transport provides a quantitative and transferable basis for studying the photochemical oxidation of isoprene and other volatile organic compounds relevant to upper-tropospheric aerosol formation.

Isoprene-derived oxygenated organic molecules (IP-OOM) drive new particle formation in the upper troposphere (Shen et al., 2024; Curtius et al., 2024), but the mechanisms that produce them, and the role of photochemistry in shaping their distribution, remain poorly constrained. These products are both hard to quantify, owing to isobaric and isotopic interferences that conventional time-of-flight mass spectrometers cannot resolve, and hard to interpret, as their formation pathways cannot be established from the measured product distribution alone. We address both with a combined measurement and modelling framework, which we demonstrate through chamber and flow reactor experiments. On the measurement side, we optimise a high-resolution Q Exactive Plus Orbitrap mass spectrometer, coupled to a bromide multi-scheme chemical ionisation inlet (the Br^–-MION2-Orbitrap), by tuning three operating parameters that change individual-species signals by 10–90 %. Benchmarked side by side against a Br^–-MION2-APi-TOF in chamber experiments, it agrees to within a factor of two for the oxygenated oxidation products while resolving isobaric interferences the time-of-flight instrument cannot. This comparison also shows that the Orbitrap’s apparent non-linearity at low concentrations, sometimes regarded as an intrinsic limitation, is rather an artefact of post-acquisition data processing: averaging that assigns zero intensity to non-detections biases the signal low near the detection limit. Disregarding the non-detections gives a conservative upper-bound detection limit of ∼1.3×10⁵ cm⁻³, comparable to that of the APi-TOF. On the modelling side, we extend MARFORCE from simple inorganic chemistry to comprehensive organic mechanisms of hundreds of species coupled to transport, and add a continuous, spatially distributed OH source; the same model can also run as a well-mixed box model for chamber experiments. Applying the framework to the OH oxidation of 1,2–ISOPOOH in the flow reactor, we detect several abundant products absent from the current isoprene mechanism, and find that photolysis of the hydroperoxide-bearing products, a previously underappreciated process, shapes the product distribution. These experiments show how pairing molecular-level measurement with a model that resolves chemistry and transport provides a quantitative and transferable basis for studying the photochemical oxidation of isoprene and other volatile organic compounds relevant to upper-tropospheric aerosol formation.

Accurate measurements of oxygenated organic molecules (OOMs) are essential to understand their relevance in secondary organic aerosol formation. Online measurements of OOMs are primarily performed with chemical ionization mass spectrometry (CIMS). These measurements often yield crowded mass spectra due to the molecular complexity of atmospheric OOMs, presenting a challenge for peak assignment and molecular identification. To investigate how mass resolution influences the interpretation of the mass spectrum, we compare three nitrate chemical ionization mass spectrometers: two atmospheric pressure interface time-of-flight (APi-ToF) instruments equipped with different ionization inlets (Eisele type and MION) and an Orbitrap with a MION-inlet. Their resolving powers are 2,500, 7,500 and 280,000 at 200 m/z, respectively. The simultaneous measurements in a Finnish boreal forest demonstrate the advantages of high resolving power for peak identification. In the mass range 260-550 m/z, the MION-Orbitrap identified 1333 non-isotopic peaks compared to the 483 non-isotopic peaks identified by the MION-ToF. By using the Orbitrap peak list, the MION-ToF could identify 1327 peaks. When measuring ions at concentrations higher than 5e5 molecules cm^-3, the instruments correlate very well, where the MION-Orbitrap and MION-ToF have an average correlation of ρ=0.77. At lower concentrations, the correlations are weak (ρ=0.42), likely due to peak overlap in the ToF mass spectra. Peak lists constructed using Orbitrap-derived molecular compositions showed clear improvements in the number of identified peaks over the conventional ToF peak list. These results demonstrate that high-resolution Orbitrap measurements can substantially enhance molecular identification in widely used ToF datasets, leading to more reliable quantification of atmospheric OOMs.

Aerosol particles play a critical role in air quality, human health, and climate processes, making their accurate characterization essential. One common way to represent aerosol populations is through particle number size distributions (PNSDs), which describe the concentration of particles across different mobility diameters. However, these distributions are often highly skewed, heavy-tailed, and multi-modal, posing significant challenges for conventional fitting approaches. In this study, we propose a robust parametric framework (AeroMiG) for modeling PNSDs using Inverse-Gamma mixture distributions. The method represents observed size distributions as a superposition of multiple Inverse-Gamma components, enabling flexible modeling of asymmetric and heavy-tailed structures that are not well captured by widely-used log-Gaussian based approaches. The performance of the proposed method is evaluated using real-world data from three measurement stations. Results are assessed using multiple criteria, including predictive accuracies, computational efficiency, and a composite score summarizing overall model quality. Comparative analysis against a widely used Aerosol Multi-mode log-Gaussian (AeroMG) demonstrates that the AeroMiG framework consistently achieves improved fitting accuracy, lower error, and faster computation. The proposed approach provides a reliable and computationally efficient solution for modeling complex aerosol size distributions, with potential applications in real-time air quality monitoring and data-driven environmental analysis.

Isoprene-derived oxygenated organic molecules (IP-OOM) drive new particle formation in the upper troposphere (Shen et al., 2024; Curtius et al., 2024), but the mechanisms that produce them, and the role of photochemistry in shaping their distribution, remain poorly constrained. These products are both hard to quantify, owing to isobaric and isotopic interferences that conventional time-of-flight mass spectrometers cannot resolve, and hard to interpret, as their formation pathways cannot be established from the measured product distribution alone. We address both with a combined measurement and modelling framework, which we demonstrate through chamber and flow reactor experiments. On the measurement side, we optimise a high-resolution Q Exactive Plus Orbitrap mass spectrometer, coupled to a bromide multi-scheme chemical ionisation inlet (the Br–-MION2-Orbitrap), by tuning three operating parameters that change individual-species signals by 10–90 %. Benchmarked side by side against a Br–-MION2-APi-TOF in chamber experiments, it agrees to within a factor of two for the oxygenated oxidation products while resolving isobaric interferences the time-of-flight instrument cannot. This comparison also shows that the Orbitrap’s apparent non-linearity at low concentrations, sometimes regarded as an intrinsic limitation, is rather an artefact of post-acquisition data processing: averaging that assigns zero intensity to non-detections biases the signal low near the detection limit. Disregarding the non-detections gives a conservative upper-bound detection limit of ∼1.3×105 cm−3, comparable to that of the APi-TOF. On the modelling side, we extend MARFORCE from simple inorganic chemistry to comprehensive organic mechanisms of hundreds of species coupled to transport, and add a continuous, spatially distributed OH source; the same model can also run as a well-mixed box model for chamber experiments. Applying the framework to the OH oxidation of 1,2–ISOPOOH in the flow reactor, we detect several abundant products absent from the current isoprene mechanism, and find that photolysis of the hydroperoxide-bearing products, a previously underappreciated process, shapes the product distribution. These experiments show how pairing molecular-level measurement with a model that resolves chemistry and transport provides a quantitative and transferable basis for studying the photochemical oxidation of isoprene and other volatile organic compounds relevant to upper-tropospheric aerosol formation.